1. Field of the Invention
The present invention relates to an ion implanting apparatus and an ion implanting method, and more particularly to such apparatus and method adapted for use in the manufacturing process of semiconductor devices.
2. Related Background Art
Ion implantation to a material with an ion implanting apparatus provides the following advantages:
1) Concentration, depth and profile of the impurity to be implanted can be determined and realized precisely, with the implanting conditions such as the ion beam current and ion energy; PA1 2) Ions can be implanted through another film; PA1 3) Selective implantation can be achieved utilizing another film, such as photoresist, as a mask; and PA1 4) A low-temperature process is possible.
For these reasons, low concentration diffusion of boron, for example, has become simplified, and such apparatus and method have become indispensable in a process of injecting an impurity into a material such as a semiconductor wafer.
The outline of the ion implanting apparatus is described, for example, in "Ion Implanting Technology" (Namba, Kogyo Chosakai Co., Ltd., P. 9-28, 1975), "Ion Implanting Apparatus" (Hashimoto et al., Special Edition of Electronic Materials, Kogyo Chosakai Co., Ltd., P. 56, 1988), "VLSI Process Apparatus Handbook" (Maeda, Kogyo Chosakai Co., Ltd., P259-272, 1990) etc.
The ion implanting apparatus is generally composed of an ion source, a mass spectrometer, an accelerator and a beam concentrating system. The mass spectrometer is used for extracting desired ions by means of a magnet, from ions generated by the ion source. The accelerator is used for accelerating the ions to a desired energy level. The beam concentrating system is used for suppressing the spreading of the ion beam.
In the implantation of accelerated ions into a material to be implanted, such as a wafer, the surface of said material tends to be charged positively, because of accumulation of positive charges as the result of emission of a large number of secondary electrons from said surface. Also the ions implanted into said material are mostly cations, which also contribute to the accumulation of positive charges. Such positive charging of the surface of the implanted material may result in electrostatic destruction of the surface or acceleration of deterioration, thus leading to a lowered production yield of the product. Such tendency is particularly conspicuous when the implanted material is completely insulating, and the significant destruction of the surface of the implanted material may hinder the preparation of the desired product.
Consequently, in the ion implantation, the prevention of charging of the surface of the implanted material, particularly the prevention of positive charge accumulation, is indispensable, and various methods have been investigated.
In practice, the following methods have been adopted for preventing the charge accumulation: (1) to reduce the amount of charge per unit area of the ion beam path; and (2) to supply electrons of lower energy. The method (1) is realized, in practice, for example by increasing the beam diameter or increasing the scanning speed. The method (2) is represented by the use of an electron shower.
However, the above-mentioned methods may be associated with the following drawbacks. The method (1) requires a long time for ion implantation, as the ion beam current cannot be made large. Also this method does not provide a complete solution to the problem, though it can suppress the charging phenomenon to a certain extent. Also, the method (2) inevitably increases the volume of the apparatus, as it requires an independent electron gun for supplying the electrons of lower energy. Also, it is difficult to neutralize the charging, on the surface of the implanted material, in two-dimentionally uniform manner, since the number of electrons (current) supplied from an electron gun is different in a central portion and in a peripheral portion of the irradiated surface of the material subjected to implantation.